In current semiconductor manufacturing technology, wide bandgap materials are typically used in optoelectronic and microelectronic devices. One such material is Gallium Nitride (GaN). GaN typically has properties such as a wide bandgap (e.g. Eg ˜3.4 eV at 300K), a large elastic modulus, high piezoelectric and piezoresistive coefficients and chemical inertness. Thus, GaN is a suitable material for microelectromechanical systems (MEMS) applications, more particularly in harsh conditions such as conditions requiring high-temperature piezoelectrics and high breakdown voltages etc.
Typically, due to the lack of GaN single crystals, GaN heteroepitaxial layers for optoelectronics and microelectronics device applications are grown on “foreign” substrates such as on sapphire (α-Al2O3) substrates having about 16% lattice mismatch, or on silicon carbide (SiC) substrates having about 3.4% lattice mismatch. It has been shown that these “foreign” substrates are typically not desirable. For example, as-grown GaN films on sapphire substrates have been shown to contain a high density of defects (e.g. mainly threading dislocations) due to the substantial lattice mismatch and thermal expansion coefficient difference between the GaN epilayers and the substrates.
Progressing from “foreign” substrates, there are other substrates, for example, silicon (Si) or silicon-on-insulator (SOI) substrates that can be used for the heteroepitaxial growth of GaN. Growth of GaN on Si (111) substrates may potentially provide an option of using silicon as a less expensive or more accessible alternative to traditional substrates. Potentially, GaN-based devices may be integrated on well-established silicon process technology. However, due to the large differences in lattice constants and thermal expansion coefficients, good quality GaN films on Si substrates cannot be obtained by metalorganic chemical vapor deposition (MOCVD). It has been proposed to use different buffer layers or intermediate layers, such as AlN and Al0.27Ga0.73N/AlN layers, to improve the GaN quality on Si. The large difference in the lattice and thermal mismatch can be avoided if suitable growth conditions are used. Apart from high-temperature AlN buffers, the method by Wang et al. described in “Effects of periodic delta-doping on the properties of GaN:Si films grown on Si(111) substrates” Appl. Phys. Lett. 85, 5881 (2004), uses suitable Si-delta-doped interlayers to reduce the cracks and tensile stress.
The integration of GaN-based devices to Si electronics can be possible if good quality wurtzite GaN epilayers can be grown on Si (100) or SOI substrates. Zhou et al, in “Comparison of the properties of GaN grown on complex Si-based structures”, Appl. Phys. Lett., 86, 081912 2005, describe GaN growth on SOI (111) that is related to integration. SOI-based technology is typically being used in microscale applications that include electronic and microelectromechanical system (MEMS) devices where single crystal silicon can offer advantages such as process control and providing reliable electronic and mechanical properties. SOI wafers can typically be prepared using methods such as wafer bonding, Smart-cut processes and the so-called Separation by Implantation of Oxygen (SIMOX) method. Using SOI substrates can provide some advantages over using sapphire or SiC substrates. The advantages include the availability of large size substrates (e.g. up to 12 in.), relatively lower cost, and easier integration with Si-based microelectronics.
M. A. Shah, S. Vicknesh, L. S. Wang, J. Arokiaraj, A. Ramam, S. J. Chua and S. Tripathy, “Fabrication of Freestanding GaN Micromechanical Structures on Silicon-on-Insulator Substrates”, Electrochem. Solid-State Lett., 8, G275 2005 describe the growth of GaN-based epilayers on (100) oriented SOI substrates prepared by the SIMOX method using MOCVD and the fabrication processes to realize GaN based micromechanical structures on a SOI platform. L. S. Wang, S. Tripathy, S. J. Chua and K. Y. Zang, “InGaN/GaN multi-quantum-well structures on (111)-oriented bonded silicon-on-insulator substrates”, Appl. Phys. Lett., 87 111908 2005 describe the growth of InGaN/GaN multiple quantum wells (MQWs) with sharp interfaces on (111)-oriented bonded SOI substrates.
One problem that may arise during development of GaN-based MEMS is a lack of efficient sacrificial etchants. Publications such as R. P. Strittmatter, R. A. Beach, T. C. McGill, “Fabrication of GaN suspended microstructures”, Appl. Phys. Lett., 78 3226 2001 and A. R. Stonas, N. C. MacDonald, K. L. Turner, S. P. DenBaars, E. L. Hu, “Photoelectrochemical undercut etching for fabrication of GaN microelectromechanical systems”, J. Vac. Sci. Technol., B 19 2838 2001 and A. R. Stonas, P. Kozodoy, H. Marchand, P. Fini, S. P. DenBaars, U. K. Mishra and E. L. Hu, “Backside-illuminated photoelectrochemical etching for the fabrication of deeply undercut GaN structures”, Appl. Phys. Lett., 77 2610 2000 describe fabrication processes of III-nitrides-based MEMS using chemical etching.
In publications such as S. Davies, T. S. Huang, M. H. Gass, A. J. Papworth, T. B. Joyce, P. R. Chalker, “Fabrication of GaN cantilevers on silicon substrates for microelectromechanical devices”, Appl. Phys. Lett., 84 2556 2004, S. Davies, T. S. Huang, R. T. Murray, M. H. Gass, A. J. Papworth, T. B. Joyce, P. R. Chalker, “Fabrication of epitaxial III-nitride cantilevers on silicon (111) substrates”, J. Mat. Sci., 15 705 2004, Z. Yang, R. N. Wang, S. Jia, D. Wang, B. S. Zhang, K. M. Lau, and K. J. Chen, “Mechanical characterization of suspended GaN microstructures fabricated by GaN-on-patterned-silicon technique”, Appl. Phys. Lett., 88, 041913 2006 and Z. Yang, R. Wang, D. Wang, B. Zhang, K. M. Lau, K. J. Chen, “GaN-on-patterned-silicon (GPS) technique for fabrication of GaN-based MEMS”, Sensors and Actuators A, Accepted for Publication (Inpress), combined dry and wet chemical etching steps have been described to realize GaN surface micromachined microstructures on Si(111) substrates. However, one disadvantage with using a wet chemical for sacrificial etching is the released microstructures must be dried in a way so as to prevent the microstructures from collapsing due to meniscus forces (stiction).
That is, the typical process to obtain free-standing surface-micromachined structures is to rinse the wet chemical etchant used to free the structures with deionized (DI) water and dry the structures using evaporation. Using this process, a flexible microstructure can be pulled down to the substrate by the capillary force of water droplets in e.g. the airgap and may remain stuck to the substrate even after the microstructure is completely dried. Studies have shown that factors such as solid bridging, van der Waals forces and electrostatic forces can give rise to stiction.
In addition to GaN, another material that is suitable for use in photonic and electronic applications is Zinc Oxide (ZnO). ZnO is typically used in a wide range of applications such as in semiconducting, photoconducting, piezoelectric sensors and optical waveguides. ZnO has a number of unique properties such as having a direct wide band gap (e.g. Eg ˜3.3 eV at 300K) and a large exciton binding energy (˜60 meV). Typically, ZnO is used for semiconductor devices operating in harsh environments, such as in space and nuclear reactors, because it is more radiation-resistive than materials such as Si, GaAs, SiC, or GaN.
For using ZnO for MEMS applications, one problem that may arise is ZnO material is easily etched by wet chemical etchants that are typically used for sacrificial etching. Thus, to realize ZnO MEMS, it is desirable to develop a dry-releasing technique.
Further to the above, yet other alternative materials for developing MEMS for use in harsh environments include microcrystalline and nanocrystalline diamond (NCD). Such materials have significant mechanical strength, chemical inertness, thermal stability and tribological performance. Freestanding NCD-mechanical structures are typically fabricated using SiO2 as a sacrificial layer. However, the SiO2 sacrificial layer is typically removed using hydrofluoric (HF) wet and/or gas etch. One disadvantage with wet chemical for sacrificial etching is the released diamond microstructures must be dried in such a way so as to prevent the structures from collapsing due to meniscus forces. This is the stiction discussed above.
Therefore, to realize e.g. wide bandgap ultra-nanocrystalline and microcrystalline diamond micromechanical structures and/or for realizing surface micromachined GaN and ZnO microstructures without stiction related problems, a dry release technique is desired.
For etching silicon, gas phase pulse etching using Xenon Difluoride (XeF2) has been used as a silicon etchant. XeF2 is a member of a family of fluorine-based silicon etchants which includes ClF3, BrF3, BrF5, and IF5. High etch rates and reaction probabilities at room temperature were found when XeF2 vapor was first used to study the mechanisms of fluorine etch chemistry on silicon. As a silicon etchant, XeF2 has unique properties such as an ability to etch without excitation or external energy sources thus exhibiting a high selectivity to many metals, dielectrics and polymers used in traditional integrated circuit fabrication, providing isotropic etching, and providing gentle dry reaction etching. XeF2 is a white solid material at room temperature and at atmospheric pressure. In a vacuum environment, solid XeF2 instantly sublimates and isotropically etches silicon without physical excitation.
Hoffman et al. in “3D structures with piezoresistive sensors in standard CMOS,” Proceedings of Micro Electro Mechanical Systems Workshop (MEMS '95), 288 1995 describe creating 3-dimensional structures with piezoresistive sensors in a standard CMOS process using XeF2 to bulk micromachine the chips. Further, U.S. Pat. Nos. 7,041,224B2, 6,942,811B2, 6,960,305 and 7,027,200B2 describe apparatus improvements (e.g. to accurately determine the end-point of the etch step) and methods used in etching of sacrificial silicon layers for a micromechanical structure (e.g. a micromirror array for a projection display and silicon-based deflectable MEMS elements) by the use of gas phase etchants, particularly in the absence of plasma (such as XeF2 with one or more diluents). Thus, silicon can be preferentially etched with respect to non-silicon materials, which include titanium, gold, aluminum, and compounds of these metals as well as silicon carbide, silicon nitride, photoresists, polyimides, and silicon oxides. Jang et al. in US20030193269A1 describe a method of forming a film bulk acoustic resonator (FBAR) having an activation area resonating with a predetermined frequency signal. The method includes forming a poly silicon layer as the sacrificial layer followed by removing the sacrificial layer using XeF2 to form a corresponding air-gap. The method further includes forming a thin layer made of dielectric material, such as AlN or ZnO, on a semiconductor substrate, such as silicon or GaAs, to generate a resonance using a piezoelectric characteristic of the thin layer.
In the above publications, for using XeF2 for dry etching, a sacrificial layer is typically deposited and patterned as additional process steps so that an airgap may be formed after the XeF2 dry etching. However, depositing and patterning of the sacrificial layer can give rise to increased complexity in the fabrication process and may incur additional cost.
Hence, there exists a need for a micromechanical structure and a method of fabricating a micromechanical structure that seek to address at least one of the above problems.